Because of recent developments in power semiconductor devices such as power MOSFETS and IGBTS, the proliferation and usage of brushless d.c. motors has intensified in recent years. These developments have enhanced the spectrum of practical uses for such motors. The applications of such motors are centered around either variable/adjustable speed or servo positioning systems. Furthermore, switched reluctance motors, for a number of reasons are particularly well suited for applications which require operation over a wide speed variation such as traction motors for electric vehicles such as automobiles, buses and trains without the use of transmissions.
The availability of high energy permanent magnets such as samarium cobalt or neodymium boron iron has also contributed to the current interest in brushless d.c. motors. Due to the high cost of these high energy magnets and mechanical difficulties of retaining them in mountings, however, there has also been a keen interest in the class of brushless d.c. motors that do not use permanent magnets or windings in connection with the rotating member--or rotor. This class of brushless d.c. motors is commonly called switched reluctance motors or simply SR motors. The design, operation, construction and use of this class of electric motors is documented in Switched Reluctance Motors And Their Control, by T. J. E. Miller, Magna Physics Publishing, 1993 ISBN1-881855-02-3.
SR motors have been used extensively as stepping motors known as variable reluctance motors. When used as stepping motors, the operation of the motor is controlled by a series of clock pulses in an open loop manner. As such, the commutation frequency and phase of the motor is driven without regard to the angular position of the rotor.
In stepping motor systems, the motor has typically been referred to as a variable reluctance (VR) motor. Many of these so-called VR stepping motors are either three-phase or four-phase machines with laminated designs having many teeth on each rotor and stator magnetic pole. The many teeth facilitate the progression of the rotor at small step angles (e.g., U.S. Pat. No. 3,866,104 to Heine). It is known to separate the plurality of phases associated with a VR stepper motor so that each stator of a plurality of stators for the motor has associated with it a single phase.
Furthermore, SR motors have been developed wherein the rotor is axially displaced from the stators. A pie-shaped rotor comprises alternating magnetically permeable/non-permeable slices separated from stator poles by axial gaps. Axial gap motors have lower power density and lower torque to inertia ratios than radial air gap motors, and are therefore not suitable for the high torque applications.
The present invention concerns a closed-loop continuously-rotating type radial gap SR motor rather than a stepping type motor which is controlled in an open-loop manner. The type of SR motor to which the present invention is directed is designed to convert electrical energy into a continuous mechanical rotation instead of bursts of torque which are difficult to control as is provided by stepping motors. The SR motor produces continuous torque (i.e., minimal torque ripple) at a desired, preset or controllable speed of motor rotation.
SR motors of the type described herein usually have stators wound with either three or four phases. Each phase is associated with a separately controlled electromagnetic winding. Each phase is energized or connected to a d.c. power source and commutated or switched at the optimum rotor position in order to produce a desired output torque characteristic having minimized torque variation as the motor rotates under the influence of the energized phases. Torque variation or torque ripple is minimized for a particular motor design at a given rotational speed by careful commutation of the phases so as to result in a constant torque vector.
In known motor control schemes, the commutation controllers energize the phases in a manner such that the duty cycles of adjacent phases overlap. The summed torque provided by the overlapping energized phases maintains the torque at a level near the peak torque for a conventional SR motor commutated with unipolar d.c. voltage.
SR motors having magnetically permeable rotors are very robust motors, have a very simple rotor construction and an extremely compactly wound stator. Such structural characteristics yield the lowest potential manufacturing cost of any known motor. Furthermore, due to their simple construction SR motors are well suited for heavy duty use in the most severe environments and can operate in temperature extremes, for example, between -100.degree. and +500.degree. C.
In SR motors there is no need for bi-directional current to energize each phase in order to produce torque since the stator poles magnetically attract soft iron rotor poles rather than north or south magnetized permanent magnets. Therefore, the direction of the current energizing the stator poles remains unchanged and the rotor poles change in accordance with the polarity of the energized stator poles.
Because polarity of the current is not important in SR motors, the stator phase windings are connected in series with switching transistors thereby eliminating the possibility of shoot-through faults. This possibility cannot be eliminated in induction motors and permanent magnet brushless motors where the phase windings are connected in a "Y" or Delta configuration.
While increasing the number of phases may reduce torque ripple, one disadvantage of increasing the number of phases and the number of poles in a motor is the increase in switching or commutation frequency. When a phase of a motor is energized or de-energized the change in magnetic flux resulting from the change in current flowing through the phase winding causes eddy current losses in the lamination iron of the stator and rotor, which in turn causes heating. As the rotation speed of the rotor increases, the commutation frequency increases for the phase windings. The increased commutation frequency increases losses which in turn causes heating in the iron core of the stator and rotor.
Another loss resulting from magnetic field flux reversals is known as hysteresis loss. These flux reversals also cause a heating loss in the iron cores of the stator and rotor and the heating effect increases with the number of phases, the number of poles and the rotational speed of the motor. A full magnetic flux reversal from a positive flux value to a negative value of flux causes a "full loop" energy loss. If the flux field only increases from zero to some maximum value and then decreases back to approximately zero when the phase winding is commutated, then a "minor" hysteresis loop is produced in the iron core.
Induction motors and permanent magnet motors require bi-directional current switching resulting in a full magnetic flux reversal in the iron core. Thus, the magnetic iron experiences heating due to full magnetic flux reversal hysteresis loops. However, SR motors, having rotors comprising magnetically permeable material and operating under uni-directional current passing through energized stator phase windings, only experience heat losses produced by minor hysteresis loops resulting from the flux linkage cycling between a near null value to a peak value and then decreasing back to the near null value. Therefore, the SR motors generally operate with less iron losses per commutation cycle than induction motors and permanent magnet motors.
SR motors having rotors comprising magnetically permeable materials are indeed desirable for their relatively lower heat losses. Nevertheless, it is desirable to further reduce the heat losses in an SR motor.
In my U.S. Pat. No. 4,883,999, an SR motor is described that significantly reduces the losses experienced in back iron of the motor. In that patent, stator windings for the same phase are located adjacently on the stator. In my U.S. Pat. No. 5,111,095, an SR motor is described that reduces the energy loss experienced in the back iron of the motor and provides for a structure wherein two phases are simultaneously energized to provide enhanced torque.